PostgreSQL is a relational database management system
(RDBMS). That means it is a
system for managing data stored in relations. Relation is essentially a mathematical
term for table. The notion of storing data
in tables is so commonplace today that it might seem inherently
obvious, but there are a number of other ways of organizing
databases. Files and directories on Unix-like operating systems
form an example of a hierarchical database. A more modern
development is the object-oriented database.

Each table is a named collection of rows. Each row of a given table has the same set of
named columns, and each column is of a
specific data type. Whereas columns have a fixed order in each row,
it is important to remember that SQL does not guarantee the order
of the rows within the table in any way (although they can be
explicitly sorted for display).

Tables are grouped into databases, and a collection of databases
managed by a single PostgreSQL
server instance constitutes a database cluster.